An important property desired in tyres is good adhesion to dry and wet surfaces. It is very difficult here to improve the slip resistance of a tyre without simultaneously impairing rolling resistance and abrasion. Low rolling resistance is important for low fuel consumption, and high abrasion resistance is the decisive factor for high tyre lifetime.
The slip resistance and the rolling resistance of a tyre depend largely on the dynamic mechanical properties of the rubbers used to construct the tyre. In order to lower rolling resistance, rubbers used for the tyre tread have high rebound resilience at relatively high temperatures (from 60° C. to 100° C.). On the other hand, rubbers which are advantageous for improving wet-slip resistance have a high damping factor at low temperatures (0° C.) and, respectively, low rebound resilience in the range from 0° C. to 23° C. To obtain compliance with this complex requirements profile, mixtures composed of various rubbers are used in the tread. The usual method is to use mixtures composed of one or more rubbers with relatively high glass transition temperature, e.g. styrene-butadiene rubber, and of one or more rubbers with relatively low glass transition temperature, e.g. polybutadiene having high 1,4-cis content or, respectively, a styrene-butadiene rubber having low styrene content and very low vinyl content, or a polybutadiene produced in solution, having low vinyl content.
Anionically polymerized solution rubbers containing double bonds, e.g. solution polybutadiene and solution styrene-butadiene rubbers, have advantages over corresponding emulsion rubbers for the production of low-rolling-resistance tyre treads. The advantages are found inter alia in the controllability of vinyl content and in the glass transition temperature associated with this, and in the branching within the molecule. The result in practice is particular advantages in the relationship between the wet-slip resistance and rolling resistance of the tyre. By way of example, U.S. Pat. No. 5,227,425 describes the production of tyre treads from a solution SBR and silica. Numerous end-group-modification methods have been developed in order to obtain a further improvement in properties, as described by way of example in EP-A 334 042 using dimethyl-aminopropylacrylamide, or as described in EP-A 447 066, using silyl ethers. However, the proportion by weight of the end groups is very small, by virtue of the high molecular weight of the rubbers, and the end groups can therefore have only a small effect on the interaction between filler and rubber molecule. EP-A 1 000 971 discloses relatively highly-functionalized copolymers containing carboxy groups and composed of vinylaromatics and of dienes, having up to 60% content of 1,2-bonded diene (vinyl content). US 2005/0 256 284 A1 describes copolymers composed of diene and of functionalized vinylaromatic monomers. The disadvantage of the said copolymers lies in the complicated synthesis of the functionalized vinylaromatic monomers and in the greatly restricted selection of the functional groups, because the only functional groups that can be used are those which do not enter into any reaction with the initiator during the anionic polymerization reaction. In particular, it is impossible to use functional groups which have hydrogen atoms which are capable of forming hydrogen bonds and which are therefore capable of forming particularly advantageous interactions with the filler in the rubber mixture.
The literature describes a wide variety of measures for the reduction of rolling resistance of tyres, and these include the use of polychloroprene gels (EP-A 405 216) and of polybutadiene gels (DE-A 42 20 563) in tyre treads composed of rubbers containing carbon-carbon double bonds. Disadvantages in the use of polychloroprene gel are found in the high price of the rubber, the high density of the polychloroprene, and the environmental disadvantages likely to arise because of the chlorine-containing component during the process for recycling of used tyres. Polybutadiene gels according to DE-A 42 20 563 do not exhibit the said disadvantages, but dynamic damping is lowered here not only at low temperatures (from −20 to +20° C.) but also at relatively high temperatures (from 40 to 80° C.), and this leads in practice to advantages in rolling resistance but to disadvantages in the wet-slip performance of the tyres. Sulphur-crosslinked rubber gels according to GB Patent 1 078 400 exhibit no reinforcing action and are therefore unsuitable for the present application.
It was therefore an object to provide rubber mixtures which do not have the disadvantages of the prior art.
Surprisingly, it has now been found that the rubber mixtures according to the invention, comprising (A) at least one functionalized diene rubber having a polymer chain composed of repeat units based on at least one diene and optionally composed of one or more vinylaromatic monomers, and (B) at least one styrene/butadiene rubber gel with a swelling index in toluene of from 1 to 25 and with a particle size of from 5 to 1000 nm and also (C) if appropriate further rubbers, fillers and rubber auxiliaries, have high dynamic damping at low temperature and very low dynamic damping at a relatively high temperature, thus giving not only advantages in rolling resistance but also advantages in wet-slip performance, and also in relation to abrasion.